# How to distinguish a spinor from a 4-vector?

Lets say we are given a four components object. To be explicit lets consider that these components are $$x^\mu = \mu$$ with $$\mu\in{0,1,2,3}$$, i.e. $$x^\mu \sim \left[ \begin{matrix} 0\\ 1\\ 2\\ 3 \end{matrix} \right] .$$ How do we know if these components represent a 4-vector or a spinor? (Forget about the 4-vector typical notation I am using. It's just the notation I choose.) I always read in books that 4-vectors (or tensors in general) are recognized by the way their components transform. Does this also apply to spinors?

Let me expand my question with an "example". Lets consider also some Lorentz transformation parametrized by $$\xi_i$$ for the "angle" of boosts and $$\theta_i$$ for the angles of rotations. Suppose we are given the components of this object after the transformation and that they are some array of numbers $$y^\mu$$. But we are not told how were they calculated or, even more interesting, both $$x^\mu$$ and $$y^\mu$$ may were measured. We thus want to relate $$x^\mu$$ and $$y^\mu$$.

Suppose that after some 'trial and error' we find that they relate by the linear transformation $$y^\mu = \left( \exp\left(\frac{i}{2} \omega_{\rho\sigma} \Sigma^{\rho\sigma}\right) \right)^\mu_{\ \ \ \nu} x^\nu$$ (which is nothing more than $$y^\mu = \Lambda^\mu_{\ \ \nu} x^\nu$$) where $$\omega_{\rho\sigma} = \left[ \begin{matrix}0 & \xi_{1} & \xi_{2} & \xi_{3}\\ -\xi_{1} & 0 & \theta_{3} & \theta_{2}\\ -\xi_{2} & -\theta_{3} & 0 & \theta_{1}\\ -\xi_{3} & -\theta_{2} & -\theta_{1} & 0 \end{matrix} \right]$$

Is it correct to conclude what follows?

1. If $$\Sigma^{\rho\sigma}$$ are given by then we conclude that the components represent a 4-vector.

2. If $$\Sigma^{\rho\sigma} = \frac{i}{4} [\gamma^\rho,\gamma^\sigma]$$ with $$\gamma^\mu$$ the Dirac matrices, or more explicitly then we conclude that the components represent a spinor.

3. If the $$\Sigma$$'s are different from these but satisfy the Lorentz algebra then the components $$x^\mu$$ represent other type of object different from a 4-vector or a spinor.

Is this correct? If yes, may this be taken as the definition of a spinor (as happens with 4-vectors) independently for they to satisfy or not Dirac equation?

• You recognize by context (possibly even by dimensional analysis). Dirac Spinor components are "internal states" and have different properties (constraints) than "just" 4-vectors. Oct 10, 2018 at 0:47
• Here's a simple explanation on Wikipedia en.wikipedia.org/wiki/Spin_representation Oct 10, 2018 at 1:10
• One way to recognize them is by the way they transform. In general, 4-vectors transform through Lorentz like $\Psi'=M(\Lambda)\Psi$, where $M(\Lambda)$ is the transformation matrix for the Lorentz transformation $\Lambda$. One way to represent this matrix is through rotation generators $M(\Lambda)=exp(\frac{i}{2}\omega^{\mu\nu}J_{\mu\nu})$. On the other hand, and spinor is an object that transform like $\Psi'=exp(\frac{i}{2}\omega^{\mu\nu}S_{\mu\nu})\Psi$, where the generators are given by $S_{\mu\nu}=\frac{i}{4}[\gamma_\mu,\gamma_\nu]$, where $\gamma_\mu$ are the Dirac gamma matrices. Oct 10, 2018 at 1:51
• When represented merely as a column vector, one cannot distinguish them - that's the point of the simple representation! As others have said above, the context should inform you of the nature of the beast. Oct 10, 2018 at 2:13
• The Casimirs of your 1 and 2 reps have very differenot eigenvalues... which? Oct 10, 2018 at 10:54